Looking Up: For some, the Big Dipper vanishes out of sight

Friday

Nothing, really, but if you were looking for it in early evening in mid-autumn, you won’t find a trace of it if you look UP. You have to look straight AHEAD - if you live in mid-northern latitudes.

To do this you also need a low, flat northern horizon. That can be tough if you live in hill country like me.

If you do have a good spot looking low in the north, the Big Dipper appears almost like it was set down on the ground, its bowl facing upward and the handle extending to the left.

Of course the Big Dipper makes this low point every 24 hours, as the Earth spins around. As October slides into November, at about 7:30 p.m. (daylight savings time), the Big Dipper is lowest in the sky. As the Earth spins and makes the sky turn, if you look out at about 3 a.m., the Big Dipper will seem to be standing up on its handle in the northeast.

The two front stars of the Big Dipper perpetually serve as a pointer to the North Star otherwise known as Polaris or Alpha Ursae Minoris.

The North Star is the only naked eye star that seems to never move, at first glance. It actually does make a small circle on the sky as the Earth turns on its axis. The axis points from the Earth’s North Pole to an imagined point in the northern sky, around which the entire sky seems to rotate once every 24 hours.

You can see this effect dramatically in a long exposure photograph in which the shutter stays open for hours to let the stars trail across.

The Earth axis very slowly wobbles, over the millennia. In fact, it takes about 26,000 years to complete one wobble. During that time, the axis inscribes an imagined circle on the sky, and whatever easily visible naked eye star is closest at that time is considered the North Star.

Alpha Ursae Minoris has been our North Star for centuries and will be for centuries more, until the axis comes anywhere near another well seen star.

It’s interesting to point a telescope anywhere in the night sky, and leave it pointing at the same spot. Look through it and see how the stars slowly move across. The higher the magnification of your eyepiece, the faster the stars seem to move across the field.

The stars move east to west across the sky, so in your eyepiece you can always tell directions by which side of the view a star sees to "enter" (from the east) or "exit" (heading west).

This is most obvious looking away from the northern sky.

Point your telescope at the North Star, and notice how very slow it seems to move. The stars in the northern sky have smaller circles to make around where the Earth’s axis points (called the "North Celestial Pole" or NCP).

The closer a star is to that point the tighter the circle.

You will also notice much dimmer stars than the North Star with a telescope, that are even closer to the NCP. These are even more deserving of North Star status, but unlike Polaris, they cannot be seen by unaided eyes, so almost no one notices.

Way to go, Polaris!

Constellations and stars that never set below the flat, northern horizon but keep missing it are termed "circumpolar".

Depending on how far you live north of the equator tells how wide an area of sky is circumpolar. The NCP, near the North Star, measures your latitude. If you are 40 degrees north of the equator, the NCP is 40 degrees high in the sky. All the stars 40 degrees around it never set below a flat horizon.

From latitude 41 degrees and further south, the the star at the tip of the Dipper’s handle (Alkaid) dips below the horizon.

As seen from latitude 29 degrees and further south, the top front star of the Dipper’s bowl (Duhbe) goes below the horizon.

Orlando, Florida, for example, is at 28 degrees north. The North Star then is very close to 28 degrees above the flat horizon. The Big Dipper is totally gone!

Last quarter Moon is on October 31.

Keep looking up!

— Peter Becker is Managing Editor at The News Eagle in Hawley, PA. Notes are welcome at news@neagle.com. Please mention in what newspaper or web site you read this column.

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